I meant to add that you’re obviously not using RAID 0 and having a setup like yours is no better, or worse than having one huge drive [from a reliability standpoint]. It all depends on what kind of risk you’re more comfortable with: a small chance of a major disaster [your one big drive dies and you lose everything], or a greater chance of a small disaster [one of four drives go and you only lose some data].
Of course you can almost eliminate all of this risk by having good backups.
That’s the main reason I do it, too, besides the added space. Doing certain tasks solely on the main drive can be SLOW.
However, of all the people I’ve spoken to, the vast majority do it because they upgraded from to a newer drive and decided to keep the older drive around for added space. I mean, heck, if you’ve got the space in your computer and your power supply can handle it, why not?
Actually, I did have RAID set up on two of my drives, and it failed, and I lost everything. So now I have the four drives, and the risks may be minimally higher, but it is definitely better to lose only a portion of the data than the whole lot. No HDD is 100% safe, I have known that from harsh experience, so I do backup, but more importantly I carefully divide up my HDD space so I do not lose everything at once, and can usually rescue stuff before the HDD fails completely by swapping between them.
It may not be mathematically in my favour to do it that way, but practically it’s the best I’ve found.
I’d just like you to know that I’m resisting the temptation to use that as my sig, but it’s not easy.